In the process of electrographic recording, a light image of an original document to be copied is recorded as a latent electrostatic image on a photoconductive member. That latent image is developed or made visible by the application of electroscopic particles or toner to the photoconductive member. The developed image may be fixed either directly upon that member or it may be transferred from that member to a recording medium and fixed to that medium. In the latter process, called transfer xerography, the recording medium is usually a plain paper sheet. Since electrographic copiers are usually of this type, we will describe the invention in that context.
In transfer xerography, the paper sheet or other recording medium carrying the developed image from the photoconductive member is routed to a fusing station in order to fuse the toner particles to the paper by the application of heat and pressure. Invariably, this involves advancing the sheet into the nip between a fuser roll and a pressure or back-up roll. The fuser roll is heated internally or externally so that its surface temperature at the nip is sufficient to fuse the toner particles to the paper sheet. The sheet carrying the fixed image is then advanced to the exit end of the copying machine.
Usually, the pressure roll in an electrographic copy machine comprises a rigid core having a resilient outer cover composed of a heat-resistant elastomeric material such as polytetrafluorethylene or room temperature vulcanizable silicone rubber (RTV).
The fuser roll which cooperates with the back-up roll to fix the toner to the paper sheet is similarly constructed with a rigid core and a cover of heat-resistant elastomeric material such as RTV. Sometimes, the fuser roll also carries an outer film of an abhesive material such as silicone oil. The oil is wiped onto the outer surface of the roll as the roll turns in the machine to inhibit the offsetting of toner from the paper sheet to the roll and to reduce the tendency of the paper to stick to the roll. Typical prior fuser and back-up rolls of this general type are disclosed, for example, in U.S. Pat. Nos. 4,372,246; 4,071,735; 3,967,042; 3,912,901 and 3,452,181.
Conventional fuser and back-up rolls, including even those provided with abhesive surface coatings or layers are disadvantaged in that they have relatively short useful lives. Over time, the heat generated at the fusing station, which may reach 175.degree. C. at the surface of the fuser roll, adversely affects the rolls there. The roll surfaces tend to deteriorate so that the printing produced on the recording medium becomes uneven and irregular in both geometry and color or blackness. In time, the surface of the fuser roll becomes degraded to the extent that toner adheres to the roll surface and often even the paper sheet itself adheres to and wraps around the fuser roll causing a paper jam. In that event, the machine usually has to be shut down and the jam relieved manually. This tends to be a tedious and time-consuming process and a source of operator annoyance. In practice, then, the end of the useful life of a roll occurs when the copy machine suffers frequent jams with the paper wrapped around the roll. It is well recognized in the industry that the available RTV rolls lose their ability to release the fused toner and paper after about 50,000 copies. This is often referred to in the manufacturers' literature as the normal replacement period of operation for such rolls.
It would be desirable therefore if there were available rolls and rollers for use particularly at the fusing station of copying machines that could withstand the high temperatures present in that operating environment in order to prolong the lives of the rolls and to minimize machine downtime.